Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk.
A conventional disk drive, generally designated 10, is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a transducer 20 mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 also contains a voice coil motor 28 which moves the transducer 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and transducer 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device.
The disk drive 10 typically includes a plurality of disks 12 and, therefore, a plurality of corresponding actuator arm assemblies 18. However, it is also possible for the disk drive 10 to include a single disk 12 as shown in FIG. 1.
FIG. 2 is a functional block diagram which illustrates a conventional disk drive 10 that is coupled to a host computer 33 via an input/output port 34. The disk drive 10 is used by the host computer 33 as a data storage device. The host 33 delivers data access requests to the disk drive 10 via port 34. In addition, port 34 is used to transfer customer data between the disk drive 10 and the host 33 during read and write operations.
In addition to the components of the disk drive 10 shown and labeled in FIG. 1, FIG. 2 illustrates (in block diagram form) the disk drive's controller 36, read/write channel 38 and interface 40. Conventionally, data is stored on the disk 12 in substantially concentric data storage tracks on its surface. In a magnetic disk drive 10, for example, data is stored in the form of magnetic polarity transitions within each track. Data is “read” from the disk 12 by positioning the transducer 20 above a desired track of the disk 12 and sensing the magnetic polarity transitions stored within the track, as the track moves below the transducer 20. Similarly, data is “written” to the disk 12 by positioning the transducer 20 above a desired track and delivering a write current representative of the desired data to the transducer 20 at an appropriate time.
The actuator arm assembly 18 is a semi-rigid member that acts as a support structure for the transducer 20, holding it above the surface of the disk 12. The actuator arm assembly 18 is coupled at one end to the transducer 20 and at another end to the VCM 28. The VCM 28 is operative for imparting controlled motion to the actuator arm 18 to appropriately position the transducer 20 with respect to the disk 12. The VCM 28 operates in response to a control signal icontrol generated by the controller 36. The controller 36 generates the control signal icontrol, for example, in response to an access command received from the host computer 33 via the interface 40 or in response to servo information read from the disk surface 12.
The read/write channel 38 is operative for appropriately processing the data being read from/written to the disk 12. For example, during a read operation, the read/write channel 38 converts an analog read signal generated by the transducer 20 into a digital data signal that can be recognized by the controller 36. The channel 38 is also generally capable of recovering timing information from the analog read signal. During a write operation, the read/write channel 38 converts customer data received from the host 33 into a write current signal that is delivered to the transducer 20 to “write” the customer data to an appropriate portion of the disk 12. As will be discussed in greater detail, the read/write channel 38 is also operative for continually processing data read from servo information stored on the disk 12 and delivering the processed data to the controller 36 for use in, for example, transducer positioning.
FIG. 3 is a top view of a magnetic storage disk 12 illustrating a typical organization of data on the surface of the disk 12. As shown, the disk 12 includes a plurality of concentric data storage tracks 42, which are used for storing data on the disk 12. The data storage tracks 42 are illustrated as center lines on the surface of the disk 12; however, it should be understood that the actual tracks will each occupy a finite width about a corresponding centerline. The data storage disk 12 also includes servo information in the form of a plurality of radially-aligned servo spokes 44 (or wedges) that each cross the tracks 42 on the disk 12. The servo information in the servo spokes 44 is read by the transducer 20 during disk drive operation for use in positioning the transducer 20 above a desired track 42 of the disk 12. Among other things, the servo information includes a plurality of servo bursts (e.g., A, B, C and D bursts or the like) that are used to generate a Position Error Signal (PES) to position the write head relative to a track's centerline during a track following operation. The portions of the track between servo spokes 44 are used to store customer data received from, for example, the host computer 33 and are referred to as customer data regions 46.
It should be understood that, for ease of illustration, only a small number of tracks 42 and servo spokes 44 have been shown on the surface of the disk 12 of FIG. 3. That is, conventional disk drives include one or more disk surfaces having a considerably larger number of tracks and servo spokes.
During the disk drive manufacturing process, a special piece of equipment known as a servo track writer (STW) is used to write the radially-aligned servo information which forms servo spokes 44. A STW is a very precise piece of equipment that is capable of positioning the disk drive's write head at radial positions over the disk surface, so that servo information is written on the disk surface using the disk drive's write head with a high degree of positional accuracy.
In general, a STW is a very expensive piece of capital equipment. Thus, it is desirable that a STW be used as efficiently as possible during manufacturing operations. Even a small reduction in the amount of data needed to be written by the STW per disk surface can result in a significant cost and time savings.
A STW is used to write servo information, by controlling the position of the disk drive's write head, on a disk surface in a circumferential fashion at each radius at which the disk drive's write head is positioned. During drive operation, the servo information is used to position the transducer of the disk drive over the appropriate data track and data sector of the disk. Accordingly, as the number of tracks per inch (TPI) increases, the amount of time necessary to write servo information increases. That is, the number of circumferential passes that a STW must make over a disk surface increases as TPI increases. Thus, unless more STWs are supplied, manufacturing times will continually increase as the TPI increases.
Instead of using a STW to write servo information in a circumferential fashion at each radius, the assignee of the present invention presently uses a STW to write servo information in a spiral fashion (in at least some of its disk drives). Specifically, the STW moves the write head in a controlled manner (e.g., at a constant velocity or along a velocity profile) from a location near the outer diameter of the disk to a location near the inner diameter of the disk (or visa-versa) as the disk spins.
FIG. 4 is a diagrammatic representation of a disk surface 210 having a first spiral of servo information 215 written thereon. The dashed line, identified by reference numeral 220, represents a track. The first spiral of servo information 215 may make multiple revolutions around the disk surface 210 (roughly two revolutions as shown in FIG. 4), but only crosses track 220 once.
FIG. 5 is a diagrammatic representation of a disk surface 210 having a first spiral of servo information 215 and a second spiral of servo information 225 written thereon. As shown in FIG. 5, the first and second spirals 215, 225 are interlaced with one another and are written approximately 180 degrees apart. Again, each spiral crosses track 220 only once.
Additional spirals of servo information may be written on the disk surface 210 depending upon the servo sample rate (that is, the number of servo samples required for each track 220 to keep the disk drive's transducer sufficiently on-track). For example, if a servo sample rate of 120 equally-spaced servo sectors per track was required, 120 equally-spaced spirals would be written on the disk surface 110. Accordingly, by writing servo information in a spiral fashion, the time necessary to write servo information on disk surface 110 using the STW is a function of the servo sample rate (i.e., the number of spirals of servo information to be written) rather than the number of tracks.
Referring again to FIGS. 4 and 5, the spirals of servo information are written by moving the disk drive's write head using the STW in a generally radial direction (more accurately, in a radial direction along an arc due to the position of the bearing assembly), while both the disk is spinning and the write head is enabled. The direction of disk rotation is indicated by an arrow as shown in each of FIGS. 4 and 5.
The disk drive's write head is enabled for nearly its entire stroke (i.e., from a position near the OD to a position near the ID or visa-versa) while under the control of the STW. As a result, a continuous spiral of servo information is written.
Each of the spirals of servo information includes sync marks written at fixed time intervals by the disk drive's write head. As mentioned above, the STW is used to move the disk drive's write head at some fixed velocity (or velocity profile) in a generally radial direction across the disk surface. If the time interval between sync marks is known and the velocity of the disk drive's write head is known, the distance between sync marks along a spiral can be determined. Specifically, the following formula may be applied: Distance=(STW Velocity)(Time), where Distance represents the radial distance between sync marks, Velocity represents the radial velocity of the disk drive's write head (under control of the STW) and Time represents the interval between sync marks.
For example, the interval between sync marks may be set at 1 microsecond, while the write head may be controlled to move at a radial velocity of 10 inches per second along its stroke. Thus, the radial distance between sync marks can be calculated to be 1 microinch along each spiral.
Each sync mark along a given spiral corresponds to a unique radius. Accordingly, the sync marks may be used to accurately position a transducer of a disk drive over the disk surface.
When writing spiral servo information onto a disk surface, the STW measures the angular position of the disk drive's actuator using an optical encoder that is concentric with the actuator's axis of rotation. The STW simultaneously tracks the amount of disk rotation using a stationary head (referred to as the clock head) to sense a timing reference track (i.e., a clock track) on the disk surface. The clock track is equivalent to an encoder for disk rotation. The process of writing spirals entails sweeping the actuator through a prescribed angle θ for a given amount of disk rotation ω while a pattern (e.g., as described above) is written by the disk drive's write head as shown in FIG. 6.
The STW also includes a digital signal processor (DSP) which, during spiral write, samples the optical encoder at a rate which is locked (via the clock track) to a set amount of disk rotation ω0. Doing so makes the desired amount of sweep per spin angle equivalent to a desired amount of sweep per sample hit θ(k). This provides a number of advantages, the most relevant being that the position profile θ(k) can be pre-calculated as a function of sample hit for any desired spiral shape. A simple case, shown in FIG. 6, is where the actuator is swept in at a constant velocity: θ(k)−θ(k−1)=θ0. As illustrated, the position of the disk drive's write head is at sample hit k=1. This position is arrived at by moving from the initial radius R(0) to radius R(1) as the disk rotates through the angle ω0.
If, after a further rotation of ω0, the actuator has swept through another increment θ0, then the disk drive's write head should arrive at the point on the spiral labeled k=2. This is illustrated in FIG. 7.
When writing spirals, the optical encoder signal is fed back and compared with a desired spiral profile at each sample hit. The error between the measured spiral profile and desired spiral profile is used by the STW servo system to compute a torque-based correction applied to the actuator. Spiral profile tracking performance and disturbance rejection are both considered in the design of the STW servo algorithm.
Spiral Runout
There are, unfortunately, disturbances during spiral writing that are not observable by the STW optical encoder used to sense actuator position. Significant among such disturbances are dimension changes in the actuator arm, disk, and push-pin damping material that are primarily due to thermal phenomena during spiral writing. These dimension changes affect the relative geometry between the disk and actuator, and thereby distort the spiral shape away from that desired. One possible manifestation of this effect is shown in FIG. 8 where the drive is shown writing the Nth spiral adjacent the 1st spiral.
Specifically, during the time interval between writing the 1st spiral and Nth spiral, the actuator pivot to write head distance has increased. The effect of this geometry-change places the Nth spiral at a distance from the 1st spiral that is greater than that desired even if the STW positioning system precisely executes the prescribed sweep angle per sample hit.
The assignee of the present invention has developed a technique for self-servo writing using the spiral servo information written onto the disk surface. In one case, the final servo patterns written by the drive appear substantially identical to traditional servo patterns.
At any given spiral servo track, correctly placed spirals exhibit an exact spiral-to-spiral spacing and the drive drive's servo system utilizes this as part of a technique to position the actuator. Spacing error of the spirals around the revolution, or spiral runout, can result in the degradation of drive position error while track following. If the spiral spacing error is extreme, the drive will fail to self-write.
FIG. 9 shows spiral runout resulting from drift in the spiral start location with respect to the disk surface. In FIG. 9, the disk has been “unwrapped” so that disk rotation/spin time is shown as increasing along the x-axis while actuator position/sweep angle increases along the y-axis. As illustrated, an outer diameter (OD) shift in spiral start is clearly seen to cause a spacing/timing shift in the spirals in the “downtrack” direction, which is constant across all radii. It should be noted that FIG. 9 illustrates the effect of a constant, incremental drift of each spiral with respect to the previous spiral.
FIG. 10 plots the resulting spiral runout (cumulative downtrack shift) due to the shift shown in FIG. 9. Each spiral starts at the same amount of offset relative to the previous spiral, but this accumulates into an increasingly larger offset from the desired location. Clearly, this causes a large discontinuity in the runout between the first-written spiral and last-written spiral, which leads to a similar, undesirable discontinuity in the drive position error signal. While the spirals can be written in a different sequence to “smooth” the discontinuity (see, U.S. patent application Ser. No. 10/788,242 entitled “Method And Apparatuses For Writing Spiral Servo Patterns Onto A Disk Surface” filed Feb. 26, 2004, which is incorporated herein by reference), the spiral runout may still have a maximum magnitude equal to the maximum offset from desired. As illustrated in FIG. 9, this runout signature will be constant at all radii when the geometrical distortion is independent of radius.
If the inner diameter (ID) drift is not the same as the OD drift, then the spiral runout is not constant across all radii. FIG. 11 illustrates this case. Here the OD and ID have drifted differently, causing both an offset and slope error in each spiral. While sequentially-written spirals will have the same discontinuous signature as before, the magnitude of the discontinuity will change from OD to ID.
In view of the above, it would be desirable to develop a method for reducing spiral runout due to, e.g., the aforementioned dimension changes.